try an early 350. they did them in 75hp guise with no intercooler
90's are ok as long as you don't load them up to much. jcb have a fleet of 90's and with all there crap in the back they really struggle up some of the hills.
i don't like 135's as the clutch feel is either on or off and so is the throttle.
my personal fave is the 125 di's such a nicer drive than the 135's imho.